A cooling tower at Disneyland was the likely source for all 22 cases in a Legionnaires’ disease outbreak last year near the theme park, an Orange County health official testified Tuesday.

Most of those who fell ill visited the park in the fall of 2017. Disneyland has denied it was the source, pointing to three infected people who had been in Anaheim but not at Disneyland. One of them died.

Dr. Matthew Zahn, medical director for epidemiology at the Orange County Health Care Agency, told an appeals board judge at the California Occupational Safety and Health Administration that those three people were in nursing homes in Anaheim. He said health workers visited the nursing homes and determined there were no likely sources of the Legionella bacteria there.

Tests around the time of the outbreak showed high levels of Legionella bacteria in two of Disneyland’s cooling towers, which likely spread contaminated droplets to people in the park, Zahn said. The medical director said he concluded the three nursing home patients were probably sickened by Disneyland as well, because water infected with Legionella bacteria “can spread two to four miles.”

Zahn pointed out that cooling towers — part of an air-conditioning system that releases mist — are the most common source of Legionnaires’ outbreaks.

Disneyland cooling tower #4 had very high levels of Legionella bacteria when people began to fall sick, Zahn said. Once it was sanitized, Legionnaires’ infections appeared to cease, he added.

“Most likely those cases were related to a common exposure,” Zahn said. “Cooling tower #4 was the most likely source of exposure.”

The health agency has never formally identified a cause of the outbreak. Upon questioning by Disneyland’s lawyers, Zahn said he could not be 100% certain that Disneyland was the source of all of the cases without additional testing.

He also said the county’s environmental health workers could not identify a source of the outbreak when they examined the park’s water sources in October last year.

“They did not find an obvious — on their pass through — source,” Zahn said.

The number of cases that occur globally is not known.[1] Legionnaires' disease is the cause of an estimated 2-9% of pneumonia cases that are acquired outside of hospital.[1] An estimated 8,000 to 18,000 cases a year in the United States require hospitalization.[10]Outbreaks of disease account for a minority of cases.[1][13] While it can occur any time of the year, it is more common in the summer and fall.[10] The disease is named after the outbreak where it was first identified, at a 1976 American Legion convention in Philadelphia.[14]

Salmonella species are intracellular pathogens;[5] certain serotypes cause illness. Nontyphoidal serotypes can be transferred from animal-to-human and from human-to-human. They usually invade only the gastrointestinal tract and cause salmonellosis, the symptoms of which can be resolved without antibiotics. However, in sub-Saharan Africa, nontyphoidal Salmonella can be invasive and cause paratyphoid fever, which requires immediate treatment with antibiotics. Typhoidal serotypes can only be transferred from human-to-human, and can cause food-borne infection, typhoid fever, and paratyphoid fever.[6] Typhoid fever is caused by Salmonella invading the bloodstream (the typhoidal form), or in addition spreads throughout the body, invades organs, and secretes endotoxins (the septic form). This can lead to life-threatening hypovolemic shock and septic shock, and requires intensive care including antibiotics.

The collapse of the Aztec society in Mesoamerica is linked to a catastrophic Salmonellaoutbreak, one of humanity's deadliest, that occurred after the Spanish conquest.

Mechanisms of infection differ between typhoidal and nontyphoidal serotypes, owing to their different targets in the body and the different symptoms that they cause. Both groups must enter by crossing the barrier created by the intestinal cell wall, but once they have passed this barrier, they use different strategies to cause infection.

Nontyphoidal serotypes preferentially enter M cells on the intestinal wall by bacterial-mediated endocytosis, a process associated with intestinal inflammation and diarrhoea. They are also able to disrupt tight junctions between the cells of the intestinal wall, impairing the cells' ability to stop the flow of ions, water, and immune cells into and out of the intestine. The combination of the inflammation caused by bacterial-mediated endocytosis and the disruption of tight junctions is thought to contribute significantly to the induction of diarrhoea.[49]

Salmonellae are also able to breach the intestinal barrier via phagocytosis and trafficking by CD18-positive immune cells, which may be a mechanism key to typhoidal Salmonellainfection. This is thought to be a more stealthy way of passing the intestinal barrier, and may, therefore, contribute to the fact that lower numbers of typhoidal Salmonella are required for infection than nontyphoidal Salmonella.[49]Salmonella cells are able to enter macrophages via macropinocytosis.[50] Typhoidal serotypes can use this to achieve dissemination throughout the body via the mononuclear phagocyte system, a network of connective tissue that contains immune cells, and surrounds tissue associated with the immune system throughout the body.[49]

Much of the success of Salmonella in causing infection is attributed to two type III secretion systems which function at different times during an infection. One is required for the invasion of nonphagocytic cells, colonization of the intestine, and induction of intestinal inflammatory responses and diarrhea. The other is important for survival in macrophages and establishment of systemic disease.[49] These systems contain many genes which must work co-operatively to achieve infection.

Salmonellosis is known to be able to cause back pain or spondylosis. It can manifest as five clinical patterns: gastrointestinal tract infection, enteric fever, bacteremia, local infection, and the chronic reservoir state. The initial symptoms are nonspecific fever, weakness, and myalgia among others. In the bacteremia state, it can spread to any parts of the body and this induces localized infection or it forms abscesses. The forms of localized Salmonella infections are arthritis, urinary tract infection, infection of the central nervous system, bone infection, soft tissue infection, etc.[52] Infection may remain as the latent form for a long time, and when the function of reticular endothelial cells is deteriorated, it may become activated and consequently, it may secondarily induce spreading infection in the bone several months or several years after acute salmonellosis.